Quality of the Matches
asks whether the matching was effective, that is, whether the matched babies were indeed comparable on the day one baby went home and the other stayed in the hospital. In some variables describe the baby and others describe the mother. In column 2 we display the 701 Early babies with characteristics noted on the day of their own discharge. In column 3 we display the matched 701 “Late” babies at the same PMA as the PMA of their Early baby matched mate when that Early baby was discharged. We aimed to have close matches here, so that we can compare the two groups, one discharged earlier than the other, but both looking physiologically similar at the PMA when the Early baby went home. To assess whether these groups were similar, we report the “DIFFAVE” results in column 5. Note that there are no significant differences between any variables described in columns 2 and 3 (no significant p-values in column 5, and no important differences in units of standard deviations). Hence, the Late baby group looked very similar to the Early group at the PMA that the Early group had when the Early group went home.
Matching Results for 701 Early and 701 Matched Late Babies
Column 4 describes what the Late group looked like when the Late group went home and column 6 describes the difference between columns 4 and 2. Of course, staying longer, the late babies were more mature on the day of their own discharge, and column (6) shows this in many ways. Here, the Late group was different in that they did not go home and instead stayed significantly longer (250.9–247.4 or approximately 3 days on average). Furthermore, the time from birth to discharge was 31 and 34 days for the Early and Late babies, respectively (results not shown). At their own day of discharge, the Late babies were older (by 3 days), had a higher propensity score hazard for discharge than when the Early babies went home (p<.0001), had a longer time interval without oxygen (a lower smoothed score) (p<.05), a longer time off gavage feeds (p<.0001), a longer time off the incubator (p<.0001), a greater combined maturity score (p<.0001), higher weight on discharge (p<.0001), a lower percent of babies discharged between 1,700 and 1,799 g, and a higher percent of patients with weight ≥1,800 g at discharge (p<.0001).
In short, the matching algorithm achieved what was desired. Late babies were comparable to Early babies when the Early baby went home, but the Late baby went home about 3 days later and were more mature on a number of dimensions.
The Day of the Week and the Discharge Rate
Why did the late baby stay longer in the hospital? There was a decreased rate of discharge on the weekend (observed number=157/day, expected=200.2/day) and an increased rate on Monday and Friday surrounding the weekend (observed=243.5/day, expected=200.2/day) (p<.0001). Hence, the maturity may not be the only factor influencing discharge—the day of the week that infants obtain maturity plays a role in determining whether such a baby may go home sooner or later.
The NICU and the Discharge Rate
There were five hospitals in our data set, all had level IIIC NICUs, all had pediatric and GYN residents, and one had residents in the NICU. There were no differences in formal discharge requirements at these institutions. There was significant variability in the rate of Early discharge between NICUs, with one NICU with as many as 65 percent of its discharges in the Early group and another NICU with as few as 37 percent (this was the NICU that utilized residents). These differences were highly significant (p<.0001 using the generalized McNemar's test), suggesting that hospital style may play a role in discharge decisions, beyond day of the week. We also constructed a conditional logistic regression model that utilized the 701 pairs of patients, and we included a variable for day of the week and distance from patient to hospital (by ZIP code centroid) and again found significant differences in the odds of being in the Early group by NICU (results not shown).
Cost and Clinical Outcomes
displays the cost and clinical outcome results comparing Early and Late matched sets. TCs were higher in those who stayed longer in the hospital. The Late group had higher TCs than the Early group, with a typical difference of $5,016 (95 percent CI $4,714, $5,235). The difference in TC was due to differences in IC (when the Early baby is already home and the Late baby is still in the hospital). ICs in the Early matched babies were small (typically $0) compared to the hospital costs in the Late matched babies (typically near $4,387 or about $1,462/hospital day). Costs subsequent to the initial period were very similar for Early and Late babies, the typical difference being about $17 for the Late–Early match. PD costs (180 days after discharge for both Early and Late) were also almost identical, with a typical difference of $12. First Costs (the costs just after discharge) for both the Late and Early babies, displayed no indication of a difference between groups. Adjusting for the individual NICU did not change the results.
Results for Cost and Coherence Outcomes
For clinical outcomes we also found no significant difference and no clinically meaningful difference between Late and Early babies. For the Late–Early matched set examining differences in outcomes PD, the typical Late baby had outcomes that ranked slightly, but not significantly, worse than the Early babies (4.3 percent worse, p=.21), so there is no sign early discharge did any harm. When examining the “First” clinical outcomes for both groups, i.e., those outcomes that occurred immediately after the discharge from the initial NICU stay, again there was no difference in median coherence scores (percent difference in ranks=0, p=.96). Again, adjusting for individual NICU did not alter our findings.
A summary of the 180-day PD results for Early as compared with Late costs and clinical outcomes (coherence scores) is displayed in . For costs and clinical coherence, the distribution of outcomes is almost identical when comparing the Early and Late babies.
Boxplots of Cost and Outcome Results by Matched Sets. PD, Postdischarge
Sensitivity Analyses on Costs
Could it be that we failed to match on some crucial covariate that was not recorded, and this covariate hides a substantial reduction in postdischarge costs in infants staying longer? We conducted a sensitivity analysis to addresses this possibility (Rosenbaum 1987
; Rosenbaum and Silber in press). The tests we constructed were equivalence tests, so they interchange the familiar null and alternative hypotheses (Berger and Hsu 1996
). The first null hypothesis we tested was whether there is actually a difference in postdischarge costs ≥|$500|, so rejecting this hypothesis of inequivalent costs provides strong evidence that the difference in costs are equivalent (defining equivalent as being <|$500|). The sensitivity analysis showed that if there were no bias from unobserved covariates, there would be overwhelming evidence (p
<.00001) that the postdischarge difference in cost is <$500. If an unobserved covariate might double the odds of discharging later, and might have a very strong relationship with postdischarge costs, the maximum possible p
-value for testing equivalency for |$500| is .0071, so such an unobserved covariate could not mislead us to think that the difference in postdischarge costs is <|$500| when this is not so. An unobserved covariate that tripled the odds of later discharge could conceivably mask a $500 difference in cost, but even one that increased the odds of later discharge by sixfold could not mask a difference in cost of |$2,500| (p
<.002), still roughly only one-half the dollar amount needed to recover the cost of delayed discharge in our study. Here, $2,500 is about half the cost of a delayed discharge.